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I'm trying to design a guitar distortion pedal, though it is not working as expected.

The built circuit behaves differently than the simulated LTspice model.

I currently do not have access to a scope, so right now I have no idea what's going wrong.

For some reason when I use the test pedal, the only distortion comes from the op-amp (clipping at the rails), however I would like for it to clip from the diodes.

I'm not too sure what's going on. If the voltage is high enough to reach the rails, it should clip from the diodes, and this is also what happens on spice.

Any help as to why this is happening would be much appreciated.

The circuit left of "in" simulates the guitar pickup, and the last resistor simulates the amplifier (roughly.)

enter image description here

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  • \$\begingroup\$ Where's your input AC coupling? What's the DC bias on amp input? It can't work unless the input DC bias is roughly half of supply voltage. \$\endgroup\$
    – Justme
    Dec 23, 2020 at 8:52
  • \$\begingroup\$ I biased the pedal Input with the 100k resistors (i think that’s right). Also, does the pedal output need to be biased? \$\endgroup\$
    – Steve
    Dec 23, 2020 at 9:13
  • \$\begingroup\$ I don't think so, the DC bias is mid-voltage only if input is shorted to ground with zero ohms source resistance. Your guitar does not have a zero-ohm source resistance, and will definitely not want DC current flowing into the pickup coil. \$\endgroup\$
    – Justme
    Dec 23, 2020 at 9:22
  • \$\begingroup\$ Oh right, that’s makes sense. What would be the best way to couple it? \$\endgroup\$
    – Steve
    Dec 23, 2020 at 9:38

1 Answer 1

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The input stage makes little sense for a guitar pickup input.

It needs to be AC coupled and then the amplifier side biased to half supply.

Note that guitar pickup input stages are somewhat special as they need to provide a certain load impedance which also means not just resistive but capacitive loading too. Due to high gain there should usually be some RF suppression.

There should be plenty of examples like guitar amps with schematics how to build a guitar input stage, but defining which is the best way to do it depends on case and opinion.

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